Spitzer Space Telescope sheds new light on 'Oumuamua ... by not being able to see it

An artist's concept of interstellar asteroid 1I/2017 U1 ('Oumuamua) as it passed through the solar system after its discovery in October 2017(Credit: European Southern Observatory / M. Kornmesser)

NASA has released new findings about 'Oumuamua, the first interstellar object to be seen visiting the Solar System. The findings based on observations by the Spitzer Space Telescope put a limit on 'Oumuamua's size, but ironically it was the inability of the instrument to detect 'Oumuamua that led to the discovery.

When 'Oumuamua (Hawaiian for "visitor from afar arriving first") was first detected on October 19, 2017 by the University of Hawaii's Pan-STARRS 1 telescope on Haleakala and then plotted as part of a NASA survey to identify near-Earth objects, it was regarded as as just another comet. However, it didn't act much like a comet and its light curve was more like that of an asteroid.

But what really caught astronomers' attention was that it was on a hyperbolic trajectory, meaning that it came from outside our system and was on its way back into interstellar space. In the year since then, 'Oumuamua has been shown to have a spindle shape and a thick coating of organic molecules, and it's been found that its orbit made slight shifts due to comet-like outgassing. In addition, there are signs that 'Oumuamua was ejected from a binary star system and has been floating in space for many millions of years.

During and after its close encounter, many telescopes, such as the Hubble and many ground observatory have turned their mirrors and lenses on 'Oumuamua, but one of the more interesting findings comes from an obvious failure. When NASA's Spitzer Space Telescope looked at the object, it couldn't see anything.

The intuitive thing should have been to just chalk the failure up to experience and leave it at that, but in science, a negative result can be as informative as a positive one. The fact that Spitzer couldn't see 'Oumuamua as it passed through the inner Solar System meant that there was an upper limit to its size. In other words, 'Oumuamua wasn't big enough for Spitzer to see it.

Looking at 'Oumuamua in the visible spectrum, scientists concluded that it had to be no larger than half a mile (2,600 feet, or 800 meters) in its longest dimension. But Spitzer works in the infrared spectrum. Using three different mathematical models based on different assumptions about the composition of 'Oumuamua, and assuming that it's a sphere for purposes of calculation, a NASA research team concluded that its actual longest dimension is between 460 and 1,440 ft (140 to 440 m) with an absolute lower limit of 320 ft (100 m).

According to NASA, this small size has large implications. For one thing, it suggests that 'Oumuamua's trajectory could be noticeably shifted by gas jets acting like natural thrusters. This means it would have to be smaller than the typical comets found in our system. In addition, 'Oumuamua may be 10 times more reflective than conventional comets – which may be due to the warming object's outgassing water vapor as it approached the Sun that then accumulated as ice and snow on the surface, making it more reflective.

"Usually, if we get a measurement from a comet that's kind of weird, we go back and measure it again until we understand what we're seeing," says Davide Farnocchia, of the Center for Near Earth Object Studies (CNEOS) at JPL. "But this one is gone forever; we probably know as much about it as we're ever going to know."